1. Field of the Invention
The present invention relates to a method for producing a coated article, particularly, to a technique in which the coated article is a planographic printing plate produced at least by coating in combination with subsequent drying a photosensitive layer coating liquid on a web on which fine asperities have been formed by surface roughening, and the coating film thickness distribution that tends to affect the sensitivity of the planographic printing plate is suppressed when the coated article is produced.
2. Description of the Related Art
In the production line of a planographic printing plate, while a web (a continuous aluminum support sheet) is being conveyed along the longitudinal direction, a coating film such as a photosensitive layer is formed on the web surface subjected to surface roughening (graining), and the coating film is dried to produce the planographic printing plate.
In this connection, there is a problem that a planographic printing plate, in particular, a multilayer CTP (computer-to-plate) plate exhibits a nonuniform sensitivity when the film thickness distribution of the coating film layer as thin as 1 g/m2 or less is nonuniform. This problem seriously affects the printing plate performance in such a way that non-negligible sensitivity differences are generated even when the film thickness distribution is caused by the asperities on the web surface created by surface roughening. This is described in more detail as follows. As shown in FIG. 5, when two layers, namely, a lower layer 2 and an upper layer 3 are formed on an aluminum support 1, the thickness of the upper layer 3 is such that the film thickness d1 of an upper layer portion 3A associated with a protruded portion 1A of the aluminum support 1 is of the order of 0.14 μm, but the film thickness d2 of an upper layer portion 3B associated with a recessed portion 1B of the aluminum support 1 comes to be of the order of 1.0 μm. Consequently, the printing durability comes to be poor in the upper layer portion 3A in which the film thickness is too thin, and spot defects due to the remaining coating film are caused by the coating film surviving the development treatment in the upper layer portion 3B in which the film thickness comes to be too thick. There is a drawback that the occurrence of such defective portions degrades the production yield. There is also a problem that such a nonuniform film thickness necessitates the coating amount to be set at an amount so as to exceed the appropriate film thickness, resulting in an increase of the cost for the coating liquid.
For the purpose of solving the above described problems, the present inventor has proposed a method in which a coating film is formed along the asperities of the aluminum support by rapidly drying in the drying step the coating film by blowing air against the coating film surface from an air nozzle disposed perpendicularly to the traveling direction of the web (Japanese Patent Application Laid-Open No. 7-89255). However, in the case of Japanese Patent Application Laid-Open No. 7-89255, high pressure air is blown perpendicularly against the coating film surface; consequently, there is a problem that the surface quality of the coating film surface comes to be degraded under the conditions that the coating film is easy to flow owing to the ample amount of the residual solvent as found in the first half of the drying step.
In this connection, the present inventor has proposed, in Japanese Patent Application No. 2003-324462, a method in which the coating film is rapidly dried in the drying step by blowing dry air along the direction parallel to the coating film surface in such a way that the surface quality is not degraded, and consequently, the coating film is formed along the asperities of the aluminum support.